It may be desirable to seal microelectronic components, such as semiconductor devices and integrated circuits (ICs), attached to printed circuit boards (PCBs) and other articles. Sealing a microelectronic component attached to a PCB can be accomplished using a variety of known methods including, for example, enclosing the microelectronic component within a physical enclosure and then sealing the enclosure, or encapsulating the microelectronic component with a liquid dielectric material that is subsequently solidified.
Unfortunately, there are drawbacks associated with these conventional methods. For example, encapsulation with a liquid dielectric material typically results in excess dielectric material which must be properly disposed of. In addition, the electrical performance of microelectronic components encapsulated in dielectric material can suffer. This is because the encapsulating dielectric material that surrounds the electrical leads of a microelectronic component can induce low signal propagation speeds. In fact, the higher the dielectric constant of the dielectric material surrounding the electrical leads, the slower the propagation speed of electrical signals.
Physical structures enclosing microelectronic components can be more desirable than encapsulation via dielectric material. One reason is because air, which has a low dielectric constant, serves as an encapsulating material within an enclosure. Because of the low dielectric constant, microelectronic components typically do not suffer from slower electrical signal propagation speeds. Another reason is that the problems associated with excess encapsulating material can be avoided by using a physical structure. Another problem is that encapsulating material and electronic components encapsulated therein can have mismatched coefficients of thermal expansion, which can result in harmful stresses in some temperature environments.
A physical structure for enclosing a microelectronic component is typically attached to a PCB adhesively or via ultrasonic welding where the enclosure walls make contact with the PCB. Ultrasonic welding involves applying a cyclic mechanical force at high frequencies that enable the creation of a bond between materials at an interface without the use of adhesives. The effectiveness of ultrasonic welding is related to the elastic modulus of the materials to be joined together. Ultrasonic welding can be limited in performance when two dissimilar materials (e.g., materials having dissimilar elastic modulus or other properties) are to be joined together. In addition, it can be difficult to apply uniform cyclic mechanical forces to the structures having multiple walls and/or complex shapes as is often encountered in microelectronics packaging.
Moreover, the application of ultrasonic waves causes components to be joined to vibrate. As such, accurate alignment can be difficult to achieve. Furthermore, because microelectronic components and PCBs often have somewhat fragile internals, they can be susceptible to damage from vibrations induced by ultrasonic waves.
Adhesive resins are often used in microelectronics packaging to seal an enclosure around a microelectronic component. However, drawbacks associated with the use of adhesive resins include expenses associated with resin storage, dispensing, and, particularly, curing. Adhesive resins can be cured at room temperature with ultraviolet (UV) light. However, the adhesive resin must be directly and completely exposed to the UV light to achieve efficient curing. Unfortunately, because of the various shapes and configurations of microelectronic components, shadow problems can prevent the UV light from reaching some portions of an adhesive resin, thereby increasing the time required to cure the resin. Moreover, curing at or below room temperature can often be a long process which can decrease production throughput and can increase production costs.
Curing adhesive resins by adding heat can reduce, often dramatically, the time required to cure. Various methods of applying heat to adhesive resin to facilitate curing are known. For example, bonding techniques utilizing induction heating techniques are described in U.S. Pat. No. 3,620,875 to Guglielmo, Sr. et al. Unfortunately, curing adhesive resins via heat can have undesirable side effects. During heating, materials with different coefficients of thermal expansion (CTE) that are being bonded together can expand at different rates which may lead to damage (e.g., warpage) to one or both of the components. In addition to CTE mismatch issues, the air inside an encapsulating physical enclosure expands during the application of heat, which increases pressure inside the encapsulating physical enclosure, which can be detrimental to the microelectronic component.
Adhesive resin curing techniques utilizing microwave energy are known and are described, for example, in U.S. Pat. Nos. 5,644,837 to Fathi et al.; 5,738,915 to Fathi et al.; 5,804,801 to Lauf et al.; 5,879,756 to Fathi et al.; and 6,312,548 to Fathi et al., each of which is incorporated herein by reference in its entirety.